Vaccines represent the most beneficial and cost effective public health measure currently known. However, as the understanding of neoplasias and infectious diseases grows, it has become apparent that traditional vaccine strategies may not be completely effective. Traditional vaccines have employed killed or attenuated organisms or antigen subunits in order to elicit immunity in an animal. A limit with these approaches, especially with killed or subunit vaccines, is that the immune response is primarily humoral in nature, and therefore not effective in combating intracellular organism or tumors that require cell mediated immunity for their destruction. Similarly, attenuated or inactivated bacteria often only induce immunization for a short period of time and immunity is limited to a humoral response. Further, traditional attenuated or inactivated bacterial vaccines do not elicit the cytotoxic T-lymphocyte (CTL) immune response necessary for the lysis of tumor cells and cells infected with intracellular pathogens.
Viral vaccines are often used to induce a CTL response in a vaccinee. Viral vaccines are usually pathogenic viruses attenuated by serial passage in cell culture or by deletion of one or more known virulence genes or viruses killed through heat or chemical inactivation. Killed viruses are incapable of infecting cells, and thus, like subunit vaccines, primarily elicit a humoral immune response. Attenuated viruses are capable of infecting cells, and can induce a CTL response in an individual. However, attenuated virus vaccines are not without drawbacks. First, attenuating a virus is often a process of trial and error. Second, there is a serious safety issue in using attenuated viruses, especially in children, the elderly, and the immuno-compromised. A solution to the problems of traditional bacterial and viral vaccines exists with bacterial vaccine vectors such as Listeria monocytogenes (LM). LM is a beta hemolytic gram positive facultative intracellular microbe.
Three methods are currently used to express a heterologous antigen in Listeria monocytogenes, and include plasmid-based expression systems and chromosome expression systems. One chromosomal based method is described in Frankel et al. (1995, J. Immunol. 155:4775-4782) and Mata et al. (2001, Vaccine 19:1435-1445). Briefly, a gene encoding the antigen of interest is placed, along with a suitable promoter and signal sequence, between two regions of DNA homologous to a region of the Listeria chromosome. This homologous recombination allows specific integration of the antigen in the Listeria chromosome. The cassette comprising the antigen and the homologous DNA is ligated into a plasmid with a temperature sensitive origin of replication which renders it incapable of replication at temperatures above 40° C. The plasmid further comprises drug resistance markers for selection and plasmid maintenance purposes. The manipulation and replication of this plasmid usually takes place in E. coli, because of its rapid replication and ease of transformation compared to Listeria. Because Listeria is a gram positive organism and E. coli is a gram negative organism, the drug resistance genes can be specific to each category of organism, or there may be two copies of the same drug resistance gene effective in both types of organism, but under the control of separate gram positive and gram negative promoters. After assembly, the plasmid is transformed into LM by direct conjugation with the E. coli comprising the plasmid, or by lysis and isolation of the plasmid from the E. coli, followed by electroporation of competent LM.
In order to integrate the plasmid into the desired region of the Listeria chromosome, the two-step allelic exchange method of Camilli et al. (1992, Mol. Microbiol. 8:143-157) is followed. Briefly, the Listeria is passaged at greater than 40° C. to prevent plasmid replication. Integration of the plasmid into the Listeria chromosome is selected by growth at 40° C. in the presence of a selecting drug, e.g. chloramphenicol. After selection of transformants, bacteria are passaged at 30° C. and selected for drug sensitivity to screen for Listeria in which excision of extraneous vector sequences has occurred. The disadvantage of this method is that the double allelic exchange method is time consuming and requires the selection of many clones in order to arrive at a suitable vaccine strain. A second chromosomal method of producing Listeria strains comprising a heterologous antigen is described by Lauer et al. (2002, J. Bacteriol. 184:4177-4186). This method does not require allelic exchange, but instead requires phage-based integration vectors. This method utilizes one or two drug resistance genes for selection of integrants, resulting in a Listeria organism comprising resistance to one or more drugs. The disadvantage of the methods of Lauer et al is the presence of drug resistance genes, which are not considered safe because of concern over the spread of antibiotic resistance from the vaccine strain to microorganisms previously susceptible to antibiotic therapy. Therefore, the presence of antibiotic resistance genes in a vaccine vector is considered undesirable from a safety perspective.
A third method of expressing foreign antigen in Listeria is to express the antigen episomally from a plasmid. This method is described in Ikonomidis et al., (1994 J. Exp. Med. 180: 2209-2218) and Gunn et al. (2001, J Immunol 167: 6471-6479). This method has the advantage that the gene does not have to be integrated into the chromosome and can be expressed in multiple copies, which may enhance immunogenicity. However, in order to select for plasmid transformants and ensure the retention of the plasmid during propagation in vitro it is necessary to include two drug resistance genes on the plasmid, one for the construction of the plasmid in E. coli and one for the propagation of the transformed Listeria monocytogenes. 
Thus, given the demonstrated uses of Listeria as a vaccine vector, methods for constructing Listeria vaccine vectors without antibiotic resistance, yet capable of eliciting a strong immune response, are needed in the field.